OBJECTIVE: The objectives of this study included a systematic review of the countries in which a seasonal pattern of preterm birth has been reported and an analysis on the seasonal variability of preterm birth in a London-based cohort. DESIGN: Cross-sectional study. SETTING: Eighteen maternity units in a London health region from 1988 to 2000. POPULATION: The study population comprised 482,765, live singleton births born after 24 weeks of gestation and weighing more than 200 g. METHODS: Systematic review and secondary analysis of seasonality over 13 years of births from the St Mary's Maternity Information System (SMMIS). MAIN OUTCOME MEASURE: Annual patterns of preterm birth and a comparison of risk by seasons. RESULTS: Three studies from developing countries and three from developed countries reported a seasonal pattern of preterm birth. One study from the USA reported no seasonal pattern of preterm birth. No British studies were located. Rates of preterm birth in developed countries were highest twice a year (once in winter and again in summer). In London (SMMIS data set), however, preterm births peaked only once a year, in winter. Babies born in winter were 10% more likely to be preterm compared with those born in spring (OR 1.10, 95% CI 1.07-1.14). CONCLUSION: Establishing a seasonal pattern of birth can have important implications for the delivery of healthcare services. Most studies from both developed and developing countries support the existence of preterm birth seasonality. This study has shown that the seasonality of preterm births in this London-based cohort differs from other developed countries that have previously reported a seasonal pattern of preterm birth.